There is a trend in the electronics industry to produce smaller devices, powered by smaller and lighter batteries. Batteries of non-aqueous electrolytic solutions in combination with a negative electrode—such as a lithium compound or carbonaceous material with lithium ions, and a positive electrode—such as lithium metal oxides can provide higher power and lower weight.
Polyvinylidene fluoride, because of its excellent electro-chemical resistance and superb adhesion among fluoropolymers, has been found to be a useful binder for forming electrodes to be used in non-aqueous electrolytic devices. U.S. Pat. No. 5,776,637 and U.S. Pat. No. 6,200,703, incorporated herein by reference, describe a PVDF binder solution in organic solvents with a powdery electrode material for use in forming an electrode to be used in a non-aqueous-type battery. JP 2000357505 describes PVDF blended with a positive active material and carbon and then dissolved in NMP as the solvent to produce a paste. The process is in line with conventional solvent casting processes for electrodes where a large amount of NMP solvent is used as a dispersion media.
The role of the organic solvent is generally to dissolve PVDF in order to provide good adhesion (non-reversible adhesion) between the powdery electrode material particles upon evaporation of the organic solvent. Currently, the organic solvent of choice is N-methyl-2-pyrrolidone (NMP). PVDF binder cast from a solvent solution provides non-reversible adhesion in electrodes and an interconnectivity of all the active ingredients in the electrode composition. The bound ingredients are able to tolerate large volume expansion and contraction during charge and discharge cycles without losing interconnectivity within the electrodes. Interconnectivity of the active ingredients in an electrode is extremely important in battery performance, especially during charging and discharging cycles, as electrons must move across the electrode, and lithium ion mobility requires interconnectivity within the electrode between powdery particles.
Unfortunately, there are several issues with these organic-solvent based binder compositions. A large amount of solvent is required for traditional electrode casting process because the slurry exhibits an abnormally high viscosity at higher concentration levels of PVDF (above 10-20 wt %), making the preparation of the electrode-forming composition difficult and the suppression of gelation of the electrode-forming composition difficult as well.
Further, the organic-solvent-based slurry presents safety, health and environmental dangers that are not present in an aqueous system. Organic solvents are generally toxic and flammable, volatile in nature, and involve special manufacturing controls to mitigate risk and reduce environmental pollution from the organic solvent. In addition, a large carbon footprint is associated with use of organic solvents that is not environmentally desirable. Further, extra manufacturing steps, costing time, money, and energy are involved to isolate PVDF formed in an aqueous media, drying the PVDF to a powder, then dissolving the powder in a solvent.
There is an environmentally-driven, and safety-driven desire to be able to produce excellent, interconnective PVDF-based electrodes, without the massive use of organic solvents.
To effectively employ waterborne slurries in electrode-forming processes, it is important to develop binder systems that are compatible with current manufacturing practices and provide desired properties of the intermediate and final products. Some common criteria include: a) stability of the waterborne fluoropolymer dispersion, having sufficient shelf-life, b) stability of the slurry after admixing the powdery material, c) appropriate viscosity of the slurry to facilitate good aqueous casting, and d) sufficient interconnectivity within the electrode which is non-reversible after drying. Additionally, from a regulatory view, fluoropolymers made without fluorosurfactants are preferred.
U.S. Pat. No. 7,282,528 entitled “electrode additive” describes fluoropolymer dispersions for cathode electrodes, which are made by using per-fluorinated surfactants. Surfactants that do not substantially remain in the electrode after drying are post-added to the fluoropolymer dispersions during concentration of the dispersion. The patent fails to teach or suggest the use of fluoropolymer made with non-fluorinated surfactant, or the use of fugitive adhesion promoters to provide interconnectivity in the electrode that is non-reversible, and exemplifies the use of only polytetrafluoroethylene (PTFE) binders, or blends of other fluoropolymers with 50% or more PTFE. The negative electrode of the examples uses a conventional solvent-based PVDF solution.
U.S. Pat. No. 7,659,335 describes similar fluoropolymer dispersions useful as electrode binders, with a specific class of non-ionic post-polymerization stabilizer. While many fluoropolymers are listed, “PTFE is preferred since melt-processing is substantially impossible”. There is no mention of fugitive adhesion promoters that could provide interconnectivity within the electrode or any other ingredients that are required. There are large differences in the properties of, processing of, and final electrodes formed from PTFE and PVDF binders. PTFE polymers have very high melting points and exhibit very strong resistance to common solvents. As a result, PTFE particles are not able to soften, flow, and adhere to powdery particles and to provide interconnectivity within an electrode. Additionally, PTFE and its blends with other fluoropolymers do not meet some of the criteria needed to form proper electrodes including stability of the waterborne fluoropolymer dispersion with sufficient shelf-life and PTFE binders do not provide sufficient interconnectivity in electrodes which is non-reversible. PVDF based binders made in accordance with this invention exhibit sufficient shelf stability, do not need concentrating steps, and as opposed to PTFE based binders, provide connectivity by adding fugitive adhesion promoters. The PVDF polymer particles are able to soften, flow and adhere to powdery materials during electrode manufacture, resulting electrodes with high connectivity that are non-reversible.
Surprisingly, a stable, aqueous electrode-forming composition has now been found for producing high quality electrodes for non-aqueous batteries and other devices having interconnectivity and irreversibility. The composition contains one or more fluoropolymers, preferably PVDF, powdery electrode material, and optionally surfactants, and fugitive adhesion promoters. Preferably the aqueous composition is free of fluorinated surfactants. The aqueous composition of the invention provides many performance, manufacturing and environmental advantages over solvent-based PVDF compositions and solvent or aqueous PTFE compositions:                a) Aqueous PVDF-based compositions are safer to use and process, less hazardous to health, and more environmentally friendly than solvent-based PVDF compositions.        b) Aqueous PVDF dispersions are advantageously synthesized using non-fluorinated surfactant.        c) Aqueous PVDF dispersions can be used as synthesized, without the need for isolating and drying to a powder, or concentration of the latex—saving time and energy.        c) Water has a lower boiling point than typically used organic solvents, so the coating can be dried at a lower temperature, or a shorter period of time, if desired.        d) Aqueous PVDF dispersions contain PVDF particles that can be softened to adhere to electrode-forming particles by using fugitive adhesion promoters—resulting in non-reversible connectivity between powdery electrode materials. The interconnectivity can be accomplished without completely coating the powdery electrode material—requiring less polymer and also creating less electrical resistance        e) PVDF-based compositions provide good connectivity, while the PTFE in PTFE-based compositions remain as discrete particles in the electrode.        f) Carbon black is easily dispersed in PVDF, but not in PTFE, and the increased dispersion increases the conductivity.        